Hydroxamic acids are organic compounds containing the ##STR1## radical and are prepared by the SchottenBauman reaction, typified by the reaction between a hydroxylamine and a propionyl chloride in an aqueous medium in the presence of an acid acceptor to form a propionamide, e.g., the reaction between a hydroxylamine acid salt (sulfate or hydrochloride) and a 3-halo-2,2-dialkylpropionyl halide such as 3-chloro-2,2-dimethylpropionyl chloride in the presence of a base to form solid 3-chloro-N-hydroxy-2,2-dimethylpropionamide as described in Example 30 of U.S. Pat. No. 4,405,357 to J. H. Chang: ##STR2## The product of this reaction (1) and a wide variety of other hydroxamic acids described in the Chang patent are intermediates for preparing various organic compounds including herbicidally active 3-isoxazolidinones, also as described in the Chang patent. The disclosure of the Chang patent is incorporated herein by reference.
Isolation of solid hydroxamic acid from the aqueous reaction medium by conventional procedures is a cumbersome, lengthy and often risky task because the product is produced as a finely dispersed solid, in many cases having the consistency of a thick slurry or paste. When it is attempted to isolate the product by batch centrifugation, for example, large portions of the solid material bypass the centrifuge, necessitating recycle of centrifugate back through the reactor. In addition, cake dewatering rates are highly variable and failure of transport and centrifuge equipment may be experienced, in turn requiring manual removal of product by operators wearing protective clothing and air masks. These problems result in significantly decreased productivity and increase the risk of contamination of work areas.
In the article "Flotation of Organic Precipitates" by T. A. Pinfold and E. J. Mahne, Chemistry and Industry, November 11, 1967, pages 1917-1918, methods for separating precipitates by flotation are described, including the flotation of insoluble, cyclic, organic compounds (such as may be contained in small amounts in a large bulk of solution) by first dissolving the compound in ethanol and then adding the alcohol solution to water. The compound then precipitates, floats and can be removed from the surface by gentle suction.
As will become apparent from the description following, the solubility of ethanol in an aqueous medium and the highly dilute character of the system prevents successful application of the Pinfold and Mahne technique to the isolation of hydroxamic acids. The technique as described in the article is thus limited to the concentrating of cyclic organic compounds in highly dilute solutions, rather than the removal of water, as in the present invention from more highly concentrated mixtures containing hydroxamic acid reaction products, resulting in efficient isolation of the hydroxamic acids.